The present study experimentally investigated the non-monotonous evolutions as well as the maximum lengths of the flame base extent (attached to the ground) and horizontal flame extent (deflected flame tip) of buoyant turbulent diffusion flames subject to relative strong cross flows of air. These transition behaviors and quantities have not been quantified in the literature, meanwhile are essential for quantifying this boundary layer diffusion combustion characteristics associated with complex fluid dynamics and air entrainment behavior, as well as for the estimation of hazardous impact of the flame upon the downstream areas. Four square burners with dimensions of 8, 10, 15 and 20 cm were used in the present work employing propane as fuel supplied with various heat release rates. The flame base extent and the horizontal flame extent were measured under cross flows provided by a wind tunnel with the cross flow air velocity with range of 0 m/s to a relative strong level, 6.0 m/s. It is shown in the results that the flame base extent and horizontal flame extent first increases with cross flow air speed at relative lower range, which is in accordance with observations reported in the literatures. However, a new phenomenon is found that once the cross flow air velocity is beyond a critical value, both these two horizontal extents of the flame decreased when the air speed is further increased. The observed two-regime behavior is then analyzed based on three dimensionless parameters including Froude number representing the counteraction of the cross flow with the buoyant flow, dimensionless heat release rate, and the ratio of gaseous fuel density to the ambient air density. New correlations are proposed to describe the two-regime evolution behavior for flame base extent and horizontal flame extent with increasing cross flow air speed, as well as maximum extents that can be reached. These new observations and proposed correlations provide an essential base to quantify the buoyant turbulent diffusion flame boundaries in relative strong wind conditions.